1. Technical Field of the Invention
The present invention relates in general to the mobile telecommunications field and, in particular, to a method and system for processing multiple random access mobile-originated calls.
2. Description of Related Art
The next generation of mobile communication systems will be required to provide a broad selection of telecommunication services including digital voice, video and data in packet and channel circuit-switched modes. As a result, the number of calls that will be made is expected to increase significantly, which will result in much higher traffic density on random access channels (RACHs). Unfortunately, this higher traffic density will also result in increased collisions and access failures. Consequently, the new generation of mobile communication systems will have to use much faster and flexible random access procedures preferably with reduced interference, in order to increase their access success rates and reduce their access request processing times.
In certain mobile communication systems, a mobile station can access a base station by first determining that the RACH is available for use. Then, the mobile station transmits a series of access request preambles (e.g., each of length 4096 chips) with increasing power levels, until the base station detects the access request. In response, the base station starts the process of controlling the mobile station""s transmitted power via a downlink channel. Once the initial xe2x80x9chandshakingxe2x80x9d between the mobile station and base station has been completed, the mobile user transmits a random access message.
More specifically, in certain Code Division Multiple Access (CDMA) systems, a mobile station will attempt to access the base station receiver by using a xe2x80x9cpower rampingxe2x80x9d process that increases the power level of each successive transmitted preamble symbol. As soon as an access request preamble is detected, the base station activates a closed loop power control circuit, which functions to control the mobile station""s transmitted power level in order to keep the received signal power from the mobile station at a desired level. The mobile station then transmits its specific access request data. The base station""s receiver xe2x80x9cdespreadsxe2x80x9d the received (spread spectrum) signals using a matched filter; and diversity-combines the despread signals to take advantage of antenna diversity.
In an IS-95 CDMA system, a similar random access technique is used. However, the primary difference between the IS-95 process and that of other CDMA systems is that an IS-95 mobile station transmits a complete random access packet instead of just the preamble. If the base station does not acknowledge the access request, the IS-95 mobile station re-transmits the access request packet at a higher power level. This process continues until the base station acknowledges the access request.
In a mobile communication system using a slotted ALOHA (S-ALOHA) random access scheme, such as the method disclosed in commonly-assigned U.S. patent application Ser. No. 08/733,501 (hereinafter, xe2x80x9cthe ""501 Applicationxe2x80x9d), a mobile station generates and transmits a random access packet. A diagram that illustrates a frame structure for such a random access packet is shown in FIG. 1. The exemplary random access packet (xe2x80x9caccess request data framexe2x80x9d) comprises one or several preambles and a message portion. In general, the preamble is a binary synchronization code with optimized autocorrelation properties resulting in the minimized probability synchronization detection at incorrect time positions.
Returning to the problems to be addressed by the present invention, as described earlier, a mobile station transmits a random access burst to access a base station. The access burst includes a preamble and a message or data part. The message part is spread by a quadriphase spreading sequence, which is also modulated so as to reduce the Peak-to-Average Power Ratio (PAPR) of the filtered transmitted signal. This same type of modulation (commonly referred to as Hybrid Phase-Shift Keying or HPSK modulation) is applied on the uplink dedicated physical channel. An important advantage of such HPSK modulation is that it allows the design of a mobile station""s power amplifier which can produce the maximum possible PAPR less 1 dB (as compared to conventional Quadrature PSK or QPSK modulation).
Alternatively, the preamble part of the transmitted random access burst is pseudo-QPSK modulated. As such, the preamble comprises a binary synchronization code that is 4096 chips long. In this case, each binary element of the code, C, is multiplied by a constant complex number:                               C          =                                    (                              1                +                j                            )                                      2                                      ,                  xe2x80x83                ⁢                  j          =                                    -              1                                      ,                            (        1        )            
just before filtering is applied in the quadrature transmitter branches. Consequently, the PAPR observed during the preamble""s transmission is 1 dB higher than the PAPR observed during the transmission of the message part (i.e., during the traffic channel transmission). The problem with this 1 dB difference in PAPRs in a burst is that it distorts the transmitted signal, which typically causes interference in neighboring frequency channels. As such, this problem is especially critical at the higher power levels, which occurs more frequently during preamble power ramping. Again, preamble power ramping is the procedure whereby a mobile station transmits successive RACH preambles at increased power levels until the base station acknowledges that a transmitted preamble has been successfully received.
Notably, the conventional HPSK modulation approach used is to map a pair of binary spreading codes into a quadriphase spreading code so that the phase differences between some successive elements of the resulting quadriphase spreading code are at most plus or minus 90 degrees. As such, it should be stressed that the phase differences of only some of the successive elements of the quadriphase code are at most plus or minus 90 degrees, because a xcfx80/2 phase restriction applies only within the blocks of N=2 chips. However, the random QPSK transition is allowed between (as opposed to within) the blocks of N=2 chips. Consequently, such random phase transitions produce (virtually) statistically-independent binary spreading sequences on the I and Q channels, which is an important condition for improved immunity against interference with QPSK spreading. Namely, HPSK modulation is a hybrid combination of xcfx80/2-biphase (BPSK) and quadriphase (QPSK) spreading which utilizes the strengths of both methods. Specifically, xcfx80/2-BPSK spreading is directed to reducing the PAPR, while QPSK spreading is directed to reducing interference. Specifically, the inter-chip interference produced by the pulse shape filtering process is reduced by half. The other-user interference (conventional multiple access interference) is independent of the relative other-user carriers"" phase.
FIG. 2 is a block diagram of a conventional HPSK modulator 100. As shown, the serial-to-parallel (S/P) conversion block 104 illustrates that the different random chips are multiplied (106) with the corresponding real and imaginary branches prior to summation (108), which produces random QPSK phase transitions after every N=2 chips. Consequently, the phase difference between the pairs of successive elements of the resulting quadriphase spreading code, Ci+jCq, is limited to a value of at most xc2x1xcfx80/2. Every other phase transition can have any value within the set {0,xc2x1xcfx80/2,xcfx80}.
Nevertheless, a significant problem with the conventional HPSK modulation approach is that it alters the correlation properties of the spreading sequences being modulated. For example, when the spreading sequence is a specially-designed synchronization code with low aperiodic autocorrelation sidelobes, after HPSK modulation has been applied, there is no guarantee that the autocorrelation properties will remain the same. Quite the opposite, usually the fidelity of the autocorrelation properties becomes much worse in such cases. Actually, this problem is the main reason why HPSK modulation is not being applied to the RACH preamble in the conventional approaches. As such, it is desirable to provide a new modulation scheme that will limit phase transitions between successive quadriphase code elements, but will produce spreading/synchronization codes having unchanged correlation properties. As described in detail below, the present invention successfully resolves this problem and other related problems.
In accordance with a preferred embodiment of the present invention, a RACH preamble modulation method for a mobile communication system is provided in which the real and imaginary components of a complex RACH preamble are filtered in corresponding pulse shaping filters. Consequently, the phase transitions are limited at all chip positions, and the resulting quadriphase codes produce transmitted signals with lower PAPRs than those transmitted with conventional HPSK modulation schemes.
An important technical advantage of the present invention is that the modulation scheme used can be applied to arbitrary binary synchronization codes, providing at most plus or minus 90 degree phase transitions at all chip positions.
Another important technical advantage of the present invention is that the modulation scheme used results in more than a 2 dB PAPR reduction of the filtered transmitted signal as compared to conventional QPSK spreading schemes.
Yet another important technical advantage of the present invention is that the modulation scheme used provides a 1 dB lower PAPR than that obtained with conventional HPSK modulation schemes.
Still another important technical advantage of the present invention is that a modulation scheme is used which produces a quadriphase synchronization code that maintains the same aperiodic autocorrelation magnitude as the original binary code used, and is also optimized to produce a minimum probability of synchronization detection at incorrect time positions.
Still another important technical advantage of the present invention is that a synchronization code receiver can be implemented by using an efficient binary synchronization code correlator.